Sounds like a good idea. Since we should prevent contamination of Mars, this would mean using waste for this is a bad idea. But maybe we can use a 250 kg tungsten cone as additional payload, this could possibly impact at 2500 m/s, giving it the power of 250 kg TNT, without the risk of spreading life from Earth.

With small engine you can add 1 or 2 km/s. I'm thinking something like this:

If it's an airless body, yes. If it has an atmosphere, we would need some sort of aeroshell. I will admit that the conical nose will not last long on entry, as the early ICBM warheads proved. However, a spherical nose in place of the point, and with a forward CG, we could be talking about something useful. However, if it takes too long, drop it and use it on another mission. If all else fails, there's a C4 charge available. Gimme a bit to find it.

However, a spherical nose in place of the point, and with a forward CG, we could be talking about something useful.

Sorry, but it looks like you are stuck in the 1950s, when ablative heat shields for nuclear warheads had been the state of the art.

The tip of the warhead is still rounded, but from a mesh making point of view, it is a cone. You can make the aft end of a solid metal impactor hollow to move the CoG forward relative to the CoP. But it isn't needed if you spin stabilize the impactor before reentry, the time in the atmosphere is too short for tumbling.